stages/disks: handle fields being pointers
stages/disks: handle fields being pointers

Fix the disks stage to handle label, start, and size being pointers.
This is be removed by later commits, but I wanted to have a testable
point first.

ajeddeloh/ignition,ajeddeloh/ignition,coreos/ignition,coreos/ignition,coreos/ignition,ajeddeloh/ignition